Apparatus for and method of providing a melted insulating coating on the inner surface of a tubular article



URGG OD QF P 3.4842 76 MELTED INSULATING A TUBULAR ARTICLE 2Sheets-Sheet 1 Dec. 16, 1969 A A. ETAL APPARATUS FOR AND E ROVIDING ACOATING ON THE NN R SURFACE OF Filed July 21, 1966 5 3 INVENTOR.

ANTHONIE .uauassmx ERNEST 0.W.VA DER JACOB w.na TE AGENT A J. BURGGRAAFET AL 3,484,276 APPARATUS FOR AND METHOD OF PROVIDING A MELTEDINSULATING COATING ON THE INNER SURFACE OF A TUBULAR ARTICLE Filed July21, 1966 r 2 Sheets-Sheet 2 QMKJW' AGENT United States Patent US. Cl.117-95 Claims ABSTRACT OF THE DISCLOSURE An insulating coating resistantto attack by corrosive vapors is applied to the inner surface of atubular article by applying a suspension of the coating material to thesurface and thereafter generating an inductive plasma which develops theheat required for melting the material.

It is known to apply a layer to the inner surface of metal vapordischarge lamps made of glass, quartz or similar substances forprotecting the wall material from attack. Such coating layers mostlyconsist of substances the melting point of which lies at a highertemperature than that of the material of which the tube wall is made.

The attack of the wall surface is due to the presence of stronglyelectropositive metal vapors such as sodium vapor, cadmium vapor,magnesium vapor, and the like and to the production of metal ions duringthe electric discharge. Since the replacement of the conventionalinexpensive kinds of glass containing or not containing lead by otherinsulating materials, for example, borate glass or boron silicate glasswhich are resistant to chemical and electrochemical action and are lesslikely to be attacked involves difliculties in processing, coatinglayers are applied consisting of the said substances. Furthermore, asatisfactory protection is obtained by coating with metal oxides capableof withstanding high temperatures and are not attacked by metal vaporsuch as alumina, zirconium oxide, calcium oxide, magnesia or berylliumoxide.

The protective action of such a coating is not only based on the kind ofmaterial used. It is important that a completely uninterrupted layershould be formed. A density such that the metal ions and the harmfulvapors absolutely cannot reach the subjacent wall cannot be attainedwith certainty if the known coating techniques are used in which thelayer is formed by sintering or burning the coating material into thesubjacent surface. It goes without saying that the formation ofcompletely uninterrupted layers is promoted by melting the appliedsubstance resistant to attack. The generally requires highertemperatures than the material for the tubular envelope is capable ofwithstanding so that such a protective layer cannot be obtained by theconventional method of application. It must be prevented that thesoftening temperature of the glass support should be exceeded during thesupply of the required heat.

The invention relates to a method of applying such a coating increasingthe resistance to chemical and electrochemical attack to the innersurface of a tubular article of glass, quartz or another insulatingmaterial, in which an uninterrupted layer is formed by melting thecoating material, while the use of insulating substances of a highermelting point than that of the tube wall does not hamper the manufactureof such a layer. According to the invention, the heat required formelting the coating material on the inner surface of the tubular articleis obice tained by the presence of an electric gas discharge in thehollow space of the article, which discharge is maintained by analternating magnetic field of high frequency produced by an inductioncoil surrounding the article.

The invention further relates to devices for carrying out the method.

In a gas discharge maintained by high-frequency induction, which isreferred to as inductive plasma, very high temperatures can be attainedwhich decrease from the core of the discharge towards the surroundings.By a suitable choice of the strength of the high-frequency inductionfield and the pressure of the gas required for the discharge, the heatrequired for attaining the desired temperature can be developed in theproximity of the wall. In the method according to the invention, use ispreferably made of a plasma at a lower gas pressure, since, whenrarefied, the gas ignites more readily and the plasma is spread on alarger sectional area. Gases having a low ionization potential bringabout a minimum of difliculties when the electric discharge is caused tocontinue so that use of preferably made of argon or neon.

/ The invention is more particularly intended for applying a thinprotective layer. With a view to the thickness of the layer, thedifference in expansion between the material of the wall of the articleand the material of the coating must frequently be taken into account.Therefore, it is sometimes preferred to apply to a first coating asecond layer, the intermediate layer consisting of another materialwhich has a coefficient of expansion lying in the transitional areabetween the expansion coeflicients of the material of the wall and thatof the protective coating.

Such an intermediate layer may be sintered, for exf ample, and may forman opaque layer if a diffuse transparency is required.

The conventional kinds of glass used for the manufacture of dischargetubes have an expansion coefiicient of from 50-75 10" cm. cm. degree C7and are not particularly resistant to the unequal expansion under theinfluence of the heating, if produced locally, for example, with the useof the inductive plasma. The risk of mechanical stresses being producedwhich exceed the strength of the material can be avoided by the use of aheating furnace by which heat is supplied from the out side to the walluntil a temperature is reached at which the material has a viscositysuitable for the neutralization of the mechanical stresses. An efiicientcombination therefore consists in the use of an elecrtic resistancefurnace and a high-frequency coil located immediately adjacent oneanother, while the tube is slipped into the furnace and is moveduniformly in the direction of the coil.

For the treatment of tubular articles of glass having an expansioncoefiicient of less than 6X10 cm. cm.- degree C.1 a separate preheatingfurnace is superfluous.

The coating material may be sucked up into the tube in the form of asuspension and the excess material can be removed after a quantity ofthis coating material has been deposited on the tube wall. The thicknessof the layer after the suspension has been conducted away depends uponthe viscosity which can be chosen in accordance with the desiredthickness of the layer. The suspension is made of powder intended forthe coating having a given grain size which is mixed with alcohol and asolution of nitrocellulose in suitably chosen quantities, whereupon themixture is ground in a ball-mill for a comparatively long time, forexample, for 24 hours.

The suspension layer can be processed to a melted layer adhering to thewall in different ways. For tubes of not too small lengths, the methodin acocrdance with the invention can be carried out in that the gas, forexample, argon is supplied at one end of the tubular article,

while the gas is conducted away at the other end by means of an exhaustdevice, the pressure in the tube being adjusted to approximately l totorr. (1 torr is a pressure of 1 mm. of Hg.) This arrangement has theadvantage that the outer wall can readily be slightly cooled in order toprevent the admissible temperature from being exceeded. The strength ofthe tube wall must remain sufficient to avoid the compression of thewall due to the decreased internal pressure. Therefore, the admissibletemperature is lower than if a Separate jacket of quartz glass oranother insulating material capable of withstanding a sufliciently hightemperature is used and the article to be treated is placed in the spacesurrounded by this jacket. Such an arrangement is more particularlyintended for the treatment of tubes of small lengths. In this case, thegas is supplied and conducted away at the ends of the jacket of quartzglass. Contamination of the argon or neon gas required for the dischargemust be avoided as far as possible so that gases must be prevented frombeing released from the suspension on the wall of the tube. The heatingof this suspension until the material melts is therefore preceded by adrying period followed by a heat treatment such that the gasformingconstitutents are removed therefrom.

The meltable layer may be applied in a manner differ ent from that inwhich a suspension is used. For this purpose, the tube is rapidlyrotated about its longitudinal axis during the activity of the inductiveplasma while simmultaneously with the gas current finely divided coatingmaterial is allowed to enter the tube. The centrifugal forces producedby the rotary movement are imparted to the powder which is heated in thegas discharge and is pushed by these forces to the outside and reachesthe wall in the melted state. The formation of a layer completelycoating the wall requires a relative displacement of the tube withrespect to the plasma and the inlet aperture of the pulverulentmaterial.

Further details of the invention will now be described with reference tothe drawing, in which:

FIG. 1 shows an arrangement for applying a coating to the walls of tubesof considerable lengths,

FIG. 2 shows such an arrangement for short tubes,

FIG. 3 shows diagrammatically a suitable device for carrving out theinvention, and

FIG. 4 shows the device for rotating the tube.

In case of not too small a length. the tube 1 may be provided at bothends with plugs 2 and 3 which both have an aperture 4 and 5,respectively. The aperture 4 in the plug 2 communicates with acontaniner containing a quantity of required gas, for example argon orneon. The aperture 5 in the plug 3 is connected with an exhaust device.The inner surface of the tube 1 is coated with a thin layer 6 of thesuspension of the material intended for the coating. Around the wall ofthe tube, provision is made of a heating furnace 7 and the inductioncoil 8 of a high-frequency generator.

In case of a tube of shorter length, a separate cylindrical jacket 9(FIG. 2) may be provided consis ing of quartz glass or of anothermaterial melting with diflicultv and having insulating properties. Theplugs 2 and 3 and the apertures 4 and 5 provided therein are disposed atthe ends of the jacket 9, while the heating furnace 7 and the inductioncoil 8 surround the jacket 9. The len h of tubing 10 to be treated isplaced inside the jacket 9, the inner surface of which length of tubingis coated with the suspension layer 6. The length of tubing 10 issupported by a tubular support 11 of material melting only withdifficulty, for example, of quartz glass.

When carrying out the method in accordance with the invention, the tube1 or the jacket 9 is placed on a platform 12 (FIG. 3). Through aflexible hose 13 the aperture 4 in the plug 2 communicates with a needlevalve 14 by means of which the supply of gas from a vessel 15 iscontrolled. The aperture 5 in the plug 3 serving for conducting away thegas communicates through a flexible hose 16 with the exhaust pump 17.With the needle valve 14 a gas pressure between 10- and 10" torr ismaintained.

The heating furnace 7 can be used for drying the suspension and servesat the same time for heating the tube glass in order to avoidimpermissibly high mechanical stresses. The furnace 7 and the jacket 9can be displaced along each other in the longitudinal direction of thetube. In order to carry out this displacement, the platform 12 isprovided with a helical rod 18 and a device cooperating therewith forconverting a rotary movement into a rectilinear movement, for example,constituted by a transmission 19 with the aid of a worm and a wormwheel,the drive being effected by the motor 20.

The voltage control member 21 of the autotransformer 22 connected to thesupply mains serves for adjusting the temperature of the heating furnace7.

Prior to the ignition of the inductive plasma, the heating furnace 7 isbrought to the correct temperature. At a low gas pressure in the tube,the movability of the gas molecules resulting from this heating issuflicient to give rise to ionization of the gas under the influence ofa high-frequency induction field. This field is obtained by theenergization of the induction coil 8 which for this purpose is connectedthrough a coupling transformer 23 to the high-frequency generator 24.

Initially the tube 1 is lowered as far as possible so that the upper endlies in the heating furnace 7. The platform 12 is set into rotation byputting the motor 20 in operation and as soon as the zone heated by thefurnace 7 has got within the reach of the induction coil 8, the coil isenergized and the gas discharge is effected. Owing to the resultantdeveloped heat, the suspension layer melts and the temperature requiredfor melting can be adjusted by control of the high-frequency energysupplied to the induction coil 8. The heat of the tube wall can besatisfactorily conducted away to the open air if a rapid cooling isdesirable for preventing softening of the glass or crystallization.

When the platform is gradually raised, the tube wall is displacedthrough the high-temperature range and provided with a coatingthroughout its length. The shorter lengths of tube of FIG. 2 are treatedin a corresponding manner and the displacement by means of the platformis adapted to the length of the tubular glass. The tube to be treated isnot subjected to a difference in pressure and the maximum permissibleviscosity can be lower without leading to deformation so that theadmissible heating temperature is higher than in the case of FIG. 1. Inorder to avoid excessive heating of the tubular glass, the heat can beconducted away more satisfactorily when a cooling gas is used which ispassed through the intermediate space between the length of tubing 10and the jacket 9.

In another method of applying a melted layer to a tube wall, a jacket 9of high melting-point insulating material is rotatably arranged betweentwo caps 25 and 26 which are rigidly connected to each other by means ofrods 27 and 28 (FIG. 4). A pulley 29 serves to set the jacket 9 intorotation. The rotatable object is supported by ball bearings 30 and 31.Gaskets 32 and 33 serve to prevent the open air from penetrating intothe gas-filled space.

The caps 25 and 26 and the interposed jacket 9 may be moved upwards anddownwards by means of, for example, a hydraulic drawing device 34 thedrawing rod 35 of which is connected through flexible cables 36 and 37to the cap 25. The induction coil 8 and the heating furnace 7 do notchange their places.

The gas supply is located in the upper cap 25, while the lower cap 26 isprovided with a connection for the outlet.

The supply duct 38 includes a thin pipe 39 which extends to the outsideand is movably arranged in a stuffing ring 40 in a closure member 41closing the supply duct 38. The pipe 39 should not change its placeduring the upward and downward movement of the jacket 9 and it shouldmerge in the proximity of the inductive plasma 42 obtained by means ofthe induction coil 8. The pulverulent material supplied through the pipe39 which serves to manufacture the coating is pushed towards the wallupon rotation of the jacket 9 and is deposited on the wall in the meltedstate owing to the heat of the inductive plasma 42. The coating may beprovidcd throughout the surface of the wall in that the construction isuniformly drawn upwards during rotation of the jacket 9 and theactivation of the plasma.

Instead of the Wall of the jacket 9, a length of tubing 43 arrangedinside this jacket and supported by a support 44 of high melting-pointmaterial may be provided on its inner surface with a coating.

What is claimed is:

1. A method of coating the inner surface of a tubular article ofinsulating material with a protective coating of a melted secondinsulating material comprising the steps of applying a suspension of thesecond insulating material to the inner surface of the article,introducing an ionizable gas in the hollow space of said tubulararticle, and subjecting the suspension to a plasma formed by ahigh-frequency induction field which develops the heat required formelting the coating material.

2. A method as claimed in claim 1, in which the discharge is effected ata gas pressure of from 10- to 10- torr.

3. A method as claimed in claim 1 in which the tubular article is movedaxially through the high-frequency in duction field to produce theplasma.

4. A method as claimed in claim 3, in which the wall of the article isheated separately.

5. A device for coating the inner surface of a tubular article ofinsulating material with a protective layer of another insulatingmaterial comprising means to connect one end of the tubular article theinner surface of which is covered with a suspension of the otherinsulating material to a gas supply conductor and the other end to anexhaust device, and an induction coil surrounding the article and beingrelatively displaceable with respect thereto in axial direction forgenerating an inductive plasma in the gas supplied to said tubularmember for melting the suspension of insulating material on the innersurface thereof.

6. A device as claimed in claim 5, in. which the tubular article isdisposed in a space within a. tubular jacket of a high melting-pointinsulating material one end of which is connected to the supplyconductor and the other end to an exhaust device, the jacket beingsurrounded by an induction coil moveable in the axial direction.

7. A device as claimed in claim 5, in which the wall of the length oftubing to be coated is arranged so as to be separately rotatable aboutthe longitudinal axis of the tube.

8. A device as claimed in claim 6, in which the wall to be coated isarranged so as to be rotatable about the longitudinal axis of the jackettogether with the surrounding tubular jacket. 9. A device as claimed inclaim 7 in which in the axis of rotation a tube of smaller diameter isprovided for supplying pulverulent coating material. from the outside toan area lying in the proximity of the plasma zone.

20. A device as claimed in claim 8 in which in the axis of rotation atube is provided for supplying pulverulent coating material from theoutside to an area lying in the proximity of the plasma zone.

References Cited UNITED STATES PATENTS 2,040,767 5/1936 Dudley 117-93.22,643,956 6/1953 Kuebler et al. 117-119.8 X 2,676,894 4/1954 Anderson etal. l17-1 19.8 X 2,940,011 6/1960 Kolb 315-111 3,179,784 4/1965 Johnson.

3,383,163 5/1968 Menashi 315-111 X RALPH S. KENDALL, Primary ExaminerCHARLES R. WILSON, Assistant Examiner U.S. Cl. X.R.

